1. Field of the Invention
The present invention relates to a method and an apparatus for displaying moving images by controlling the levels of gradations of a matrix of display pixels on a display panel with drive pulses, and more particularly to such a method and an apparatus for displaying moving images while correcting false moving image contours.
2. Description of the Related Art
One of various display units capable of displaying moving images is a plasma display panel. The plasma display panel displays images through emission of light from phosphors based on electric discharges, and is expected to attract much attention as a panel-type display unit which emits light on its own with high luminance.
There are basically two types of plasma display panels, i.e., a DC (Direct Current) plasma display panel and an AC (Alternating Current) plasma display panel. Since the AC plasma display panel has electrodes not exposed in a discharge space, the AC plasma display panel is said to be more durable than the DC plasma display panel where electrodes are exposed in a discharge space.
AC plasma display panels are classified into opposed discharge design and surface discharge design. The opposed discharge structure has vertical and horizontal electrodes facing each other across a discharge space. The surface discharge structure has pairs of surface discharge electrodes comprising scanning and sustaining electrodes, which are disposed on a flat surface.
AC plasma display panels are promising as large-size full-color flat display panels because they provide a large memory margin and have good light emission efficiency.
One conventional AC plasma display panel will be described below with reference to FIG. 1 of the accompanying drawings.
As shown in FIG. 1, a plasma display panel 100 comprises a plurality of surface discharge electrodes 101 extending parallel to rows and spaced successively along columns. Each of the surface discharge electrodes 101 comprises a scanning electrode 102 and a sustaining electrode 103 which are disposed parallel to each other.
A plurality of data electrodes 104 extending parallel to the columns are disposed in opposed relation to the surface discharge electrodes 101. The data electrodes 104 are spaced successively along the rows.
Discharge spaces 105 filled with a discharge gas such as of helium, neon, xenon, or the like are provided between the electrodes 101, 104, forming display cells capable of individually emitting light at the points of intersection between the surface discharge electrodes 101 and the data electrodes 104.
The scanning and sustaining electrodes 102, 103 are disposed as electrical conductive thin films on a glass substrate 106. The data electrodes 104 are printed as electrical conductors on another glass substrate 107.
A white glazed layer 108 is deposited on the data electrodes 104 and positioned underneath a plurality of partitions 109 extending parallel to the columns and spaced successively along the rows.
Gaps defined between the partitions 109 are disposed as the discharge spaces 105 in opposed relation to the data electrodes 104. Phosphor layers 110 are coated on surfaces which define the discharge spaces 105.
A dielectric layer 111 is positioned in facing relation to the surface discharge electrodes 101.
A plurality of data drivers are connected respectively to the data electrodes 104, and a plurality of scan drivers are connected respectively to the scanning electrodes 102.
One or more sustain drivers are connected to the sustaining electrodes 103. These drivers jointly make up a driver circuit (not shown) for the plasma display panel 100.
The display pixels arranged in a two-dimensional matrix on the screen of the display panel are individually controlled for light emission to display desired images in dot matrix patterns.
A process of displaying an image on the plasma display panel 100 shown in FIG. 1 will be described below.
In a preparatory mode, preliminary discharge pulses are applied between the scanning electrodes 102 and the sustaining electrodes 103 of the plasma display panel 100 to produce preliminary discharges between those electrodes. With the preliminary discharges thus produced, discharges will stably be developed in the plasma display panel 100 for displaying images.
Then, the scan drivers apply progressively shifted scanning pulses respectively to the scanning electrodes 102, and the data drivers apply data pulses to certain data electrodes 104 which correspond to an image to be displayed, in synchronism with the scan drivers. The positions of all display pixels are progressively scanned to write wall charges in those display pixels which correspond to the image to be displayed.
Then, sustaining pulses are applied as drive pulses to all the scanning electrodes 102 and all the data electrodes 104. Then, only the phosphor layers 110 of the display pixels in which the wall charges have been written emit light, displaying a dot matrix of image with binary values on the plasma display panel 100.
There has been a demand for the display of images in multiple gradations on the plasma display panel 100. One process for meeting such a demand is a subfield process.
The subfield process will be described below. The display pixels which correspond to the image to be displayed emit light when sustaining pulses are applied with the wall charges being written in those display pixels. Therefore, the luminance of emitted light can be adjusted when the number of applied sustaining pulses is controlled.
One frame which represents a unit of time for displaying images is divided into a plurality of subfields, and sustaining pulses are established in advance as drive pulses of various durations for those subfields.
For example, if a video signal is to be represented in 256 8-bit binary gradation levels, then, as shown in FIG. 2a of the accompanying drawings, there are established subfields in one frame which serve as sustained emission periods for applying sustaining pulses at the ratio of xe2x80x9c1, 2, 4, . . . , 128xe2x80x9d.
By appropriately combining the sustaining pulses in those subfields, it is possible to change the number of sustaining pulses in one frame within the range of 256 pulses. Therefore, the matrix of display pixels on the plasma display panel 100 can be energized in a time-division multiplex fashion.
For example, if the gradation level of a certain display pixel is xe2x80x9c127xe2x80x9d, then, as shown in a left-hand side of FIG. 2b of the accompanying drawings, trains of sustaining pulses in 7 subfields that are weighted respectively by xe2x80x9c1, 2, . . . , 64xe2x80x9d are applied to the display pixel. Consequently, 7 trains of sustaining pulses which are weighted by xe2x80x9c127xe2x80x9d are applied to the display pixel in the period of one frame.
If the gradation level of a certain display pixel is 128, then, as shown in a right-hand side of FIG. 2b, sustaining pulses of one subfield which is weighted by xe2x80x9c128xe2x80x9d are applied to the display pixel in the period of one frame.
When the plasma display panel 100 is energized according to the subfield process, since the number of sustaining pulses applied in one frame to display pixels on the plasma display panel 100 can be adjusted, displayed images can be expressed in gradations.
However, some moving images displayed according to the subfield process tend to suffer interferences.
For example, when an image whose lightness varies smoothly, e.g., an image of a cheek of a human""s face, moves on the display screen, a dark or bright contour may appear in an image region which should be smooth.
When a color image is displayed, it may suffer a color-shifted contour or a reduction in resolution.
Such interferences are referred to as false moving image contours.
In a displayed color image, since bit carry points for the respective colors are spatially different from each other, interferences occur at different positions with respect to the respective colors.
While these interferences may be referred to as false color contours, they are essentially generated by a combination of false moving image contours for the respective colors in a displayed color image.
Such a phenomenon is responsible for color shifts or reductions in resolution in the display of moving images.
FIG. 2b illustrates a situation in which the gradation level of a certain display pixel varies from xe2x80x9c127xe2x80x9d to xe2x80x9c128xe2x80x9d. In the gradation level of xe2x80x9c127xe2x80x9d, sustaining pulses are concentrated in the first half of the frame. In the gradation level of xe2x80x9c128xe2x80x9d, sustaining pulses are concentrated in the second half of the frame. Therefore, a blank period free of sustained emission is present between frames across a transition from the gradation level of xe2x80x9c127xe2x80x9d to the gradation level of xe2x80x9c128xe2x80x9d. Because the display pixel does not emit light for a period of time longer than the preceding and following frames, the gradation level of the display pixel which is actually visually perceived by the human eye is lower than the gradation level that is to be displayed.
Conversely, when the gradation level of a certain display pixel varies from xe2x80x9c128xe2x80x9d to xe2x80x9c127xe2x80x9d, as shown in FIG. 3 of the accompanying drawings, the gradation level of the display pixel is actually visually perceived as being lower than the gradation level that is to be displayed because the light emission in subfields is concentrated in a short period of time.
For varying the gradation level of a display pixel on a CRT (Cathode-Ray Tube) display unit, the intensity of the electron beam may be modulated to adjust the luminance of the display pixel in an analog fashion. A number of display pixels on the CRT display unit are successively scanned in order to display an image on the CRT display unit. Since the successive scanning of the display pixels is completed in an instantaneous period of time, the CRT display unit does not suffer false moving image contours.
In apparatus for displaying moving images according to the subfield process, such as plasma display panels, each gradation bit is displayed in a time-division multiplex fashion at a low speed in a period of time close to the duration of one field, and the observer visually combines displayed gradation bits as one image based on the spatial integration performed by the human eye.
If a visually combined image is a moving image, then a clear bright-line interference or dark-line interference occurs when the moving image is followed by the eye. Specifically, when pixels visually perceived as bright or dark pixels are followed by the eye, they are combined as a bright or dark line fixedly on the retina.
The principles of generation of false moving image contours have been described above.
Processes for eliminating false moving image contours have been disclosed in Japanese Patent Laid-Open Publication No. 271325/95, Japanese Patent Laid-Open Publication No. 54852/96, and Japanese Patent Laid-Open Publication No. 234694/96, for example.
According to the processes disclosed, combinations of gradation data which make actually displayed gradations inappropriate are registered in advance. If gradation data of a preceding frame and gradation data of a present frame match any of the registered combinations, then sustaining pulses to be outputted for the gradation data of the present frame are corrected to a predetermined form.
Therefore, display pixels whose gradation levels vary according to any of the registered combinations of gradation data are supplied with sustaining pulses that have been corrected to make displayed gradations appropriate. As a result, a moving image is prevented from suffering a false contour such as a bright line or a dark line.
Moving image display apparatus revealed in the above publications register combinations of gradation data which make actually displayed gradations inappropriate in advance, and correct sustaining pulses to prevent the gradations of display pixels from being inappropriate when any of the registered combinations is detected.
Actually, however, it is difficult to optimally correct the displayed levels of multiple gradations. Particularly, it is difficult to thoroughly eliminate inadequately colored lines from fully colored images. For example, while it is possible to produce corrective settings for sufficiently eliminating false contours from images which are moving at a certain speed, the levels of such corrective settings will be improper, tending to generate bright and dark lines adjacent to each other, when images are moving at speeds higher and lower than the expected speed.
It is therefore an object of the present invention to provide a method and an apparatus for displaying moving images while appropriately correcting false contours of the moving images.
A method according to the present invention displays a moving image by dividing a frame into a plurality of subfields having different relative luminance ratios and displaying a moving image of multiple gradations on a display panel having a matrix of display pixels.
The method comprises the steps of producing new n (n is a natural number) corrected video gradation data from video gradation data of a preceding frame and video gradation data of a present frame for each of the display pixels, selecting at least two of the video gradation data of the preceding frame, the video gradation data of the present frame, and the n corrected video gradation data, and displaying the selected video gradation data at display pixels of a predetermined selection pattern.
According to this method, the gradation levels of display pixels in a region where a false moving image contour occurs are not corrected uniformly, but excessively corrected display pixels and uncorrected display pixels are mixed together in a two-dimensional pattern. Therefore, the false moving image contour is effectively prevented from occurring.
An apparatus for displaying a moving image according to the present invention includes data input means, data storage means, and data correcting means. The data input means enters data, and the data storage means stores video gradation data of a preceding frame corresponding to unit pixels of the display pixels. The data correcting means produces new n (n is a natural number) corrected video gradation data from the video gradation data of the preceding frame stored in said data storage means and video gradation data of a present frame from the data input means. The apparatus divides a frame into a plurality of subfields having different relative luminance ratios and displays a moving image of multiple gradations.
The apparatus also has correction control means. The correction control means combines a plurality of video gradation data including at least two of the video gradation data of the preceding frame, the video gradation data of the present frame, and the n corrected video gradation data, and disperses the combined video gradation data in a pixel plane of the display pixels according to a predetermined selection pattern.
The display pixels arranged in a matrix normally display images at gradation levels corresponding to video gradation data. If a combination of gradation data of a preceding frame and gradation data of a present frame is such that it generates a false moving image contour, then the gradation data is corrected to prevent the false moving image contour from occurring.
Since the degrees of data conversion for the respective display pixels are dispersed according to a selection pattern, the gradation levels of display pixels in a region where a false moving image contour occurs are not corrected uniformly, but excessively corrected display pixels and uncorrected display pixels are mixed together in a two-dimensional pattern.
With the above apparatus, because the gradation levels of display pixels in a region where a false moving image contour occurs are not corrected uniformly, but are corrected such that excessively corrected display pixels and uncorrected display pixels are mixed together in a two-dimensional pattern. Consequently, it is possible to effectively prevent a false moving image contour from occurring.
The apparatus for displaying moving images according to the present invention has a matrix of display pixels on a display panel for displaying gradations according to the subfield process. The apparatus may comprise a plasma display apparatus or a DMD, for example, as long as it can display moving images on its display panel.
The means referred above may be arranged in any way that allows them to perform their functions, and may be a dedicated hardware arrangement, a computer programmed to perform the functions, functions implemented in a computer by an arbitrary program, or any of combinations thereof.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.